1. Field of the Invention
The present invention relates to visibility measuring systems and to improvements therein for increasing the visibility range measuring rate.
2. Description of the Prior Art
Visibility measuring systems are well known for their applicability in measuring with precision the visibility range in particular mediums at particular times and locations, and such systems have found frequent use at locations such as airfields. Different structural set-ups for such measuring systems are known, and one such set-up is shown in U.S. Pat. No. 3,694,936 wherein a radiation source is used to emit light beams towards a series of reflectors which are positioned along an essentially straight-line path from the radiation source at predetermined distances, and a receiving unit is utilized to receive the reflected beams (echoes) from the reflectors and to count the echoes which have a certain retained amplitude. By counting the number of echoes from the individual reflectors which have a certain minimum predetermined amplitude, the necessary information is obtained for determining the instantaneous distance of visibility (visibility range) in the air medium.
As is shown in U.S. patent application Ser. No. 559,049, now abandoned, an improved visibility measuring system includes a radiation source (such as a laser (light) emitter), a number of reflectors arranged at varying predetermined distances from the radiation source and with a laterally spaced-apart relationship from each other, and a receiver unit which is positioned to receive and detect reflected radiation from each individual reflector as it is sequentially illuminated by the radiation source. During the measuring cycle, each reflector is illuminated by the radiation source and the reflected radiation then measured, starting generally from the reflector located farthest from the radiation source, until all the reflectors have been illuminated. The number of reflectors which are "visible" to the receiver during the measuring cycle allows for a determination of the visibility range, this visibility range being in fact equal to twice the distance between the radiation source and the appropriately utilized reflectors (since with the receiver being attached to the radiation source, the radiation received by the receiver will have passed twice the distance between the radiation source and the reflectors).
With respect to the system in U.S. patent application Ser. No. 559,049, since the radiation source is directed towards and illuminates each reflector for a certain time interval for each measuring cycle, measurements are made with respect to all of the reflectors whether or not such is necessary to indicate the range of visibility at a particular time. Thus, if the visibility range is 500 meters at a particular time, the reflector located at a distance of 250 meters from the radiation source/receiver will be the farthest reflector from which radiation of a countable amplitude will be perceived. However, even though the reflectors then farther away will be in effect inoperable, i.e., since they will not reflect radiation sufficiently to have a countable amplitude, according to the prior art they will nevertheless be sequentially radiated during each measuring cycle, as will the nearby reflectors when (at a different time) the visibility range is quite far away. This results in the apparatus performing multiplicity of unnecessary function and thus in a reduction in the optimum visibility range measuring rate.
It is an object of the present invention to provide an improved visibility measuring system wherein the measuring rate can be increased by elimination of the unnecessary radiations and measurements of reflectance against reflectors that are irrelevant to a determination of the particular visibility range at a particular time.